I like the mids and up of the rca pair but the off-center presentation that changes sides when tubes change positions, bothers me, especially when paying a premium for matching.
Anyhow, I've been in contact with UTracer. May be posting a diy tube tracer build before too long.
@Tom_S said:
I think you could put a 400Hz sign wave into the preamp and see if you have any pairs with the same voltage at the output. Just make sure the input level doesn't change and it's the same on both grids. That will probably get us fairly close.
Tom, you would probably need to put a 1k ohm dropping resistor in series with each input to insure that the mono 400Hz signal coming from the source drops out evenly across both input channels. I tested this and found that this was necessary to insure proper voltage breakouts. I have been using both channels of my oscilloscope to measure and view the relative voltage match at the outputs, but you could also use a pair of DMM's to do this.
I have now done a bunch of transconductance (GM) measurements on my tube tester and I am comparing the results (matches?) with the simple voltage gain matching procedure that you suggested above. And I am coming to the conclusion that transconductance matching is NOT a very good matching procedure at all. You also need to take into account the tubes native voltage gain (mu), and the tube's heater filament resistance. Heater filament resistance must be taken into account because the filaments are connected in series on this particular 6J1 preamp build. One tube could be getting 5.7 volts while the other is getting 6.5 volts. If you have ever tested any tubes on a tester, you know that filament voltage can make a huge difference in the transconductance measurement!! And you don't know this breakout unless you measure the filaments actually connected in series in the preamp. A tube tester does not do this. Your simple, in circuit, voltage gain measurement procedure may actually be the best way to match these tubes. Here are a couple links I found on this subject that reach the same conclusion:
Strange... This design seems a lot more sensitive to slight tube variances than I'm use to. Grant it I've only built a few preamps, line stages, and power amps. But I've never experienced significant stage shift with tubes not matching perfectly. On the contrary I have had major stage shifts with tweeters that were poorly matched (dB wise). I wonder what makes this circuit design so picky.
Around the 8:20 mark, 3 tubes are tested from 12v - 240v. While they tend to be fairly close at higher voltages, not so much at 12v. Maybe this helps explain why these preamps seem to be so picky about matching?
I started wondering what it would take to bump up the voltage. Omitting the doubler circuit for starters. It looks like there are other kits out there tailored to higher voltages (PRT-01A). So starting to get out of scope.
On a side note it does look like Zero Zone has someone with some audio experience designing their boards. All the boards I've seen seem to incorporate star grounding for the signal stuff then switch to power plane style design for the power supply sections.
I started wondering what it would take to bump up the voltage. Omitting the doubler circuit for starters. It looks like there are other kits out there tailored to higher voltages (PRT-01A). So starting to get out of scope.
On a side note it does look like Zero Zone has someone with some audio experience designing their boards. All the boards I've seen seem to incorporate star grounding for the signal stuff then switch to power plane style design for the power supply sections.
You may be onto something about the low voltages. The 6AK5 is designed to operate at a much higher plate voltage (120Vdc or so) and we are only using 60Vdc. Plate current is only about 2.5ma, whereas at higher voltages it is about 7ma. My tube tester is probably measuring the transconductance at a much higher voltage. Best procedure for matching, going forward, will probably be to use a combination of the transconductance measurements AND the actual "in-circuit" gain measurements made at the lower voltages. The utracer that Kornbread is looking at could simultaneously measure both gain and transconductance at the lower voltages by simply programming the curves for the proper set of voltages. My Heathkit tube tester runs the test at one and only one operating point and I cannot adjust this. The problem with the utracer, however, is that construction of a device like this looks like a very advanced technical project with lots of twists and turns and a low probability of success.
I have the 6J1/5654 model set up in LTspice, so I could adjust the plate voltage to 120 and re-set the operating point for an idle current of around 6 to 7ma and a closed loop gain of about 8dB or so. But this would be a major project and would involve a complete redesign of both the PC board and parts list. Caps would all have to be upgraded to around 200V or so. I have searched around the web looking for someone that has already done this, but came up empty. Do you have a link to an available design?
@4thtry said:
I have the 6J1/5654 model set up in LTspice, so I could adjust the plate voltage to 120 and re-set the operating point for an idle current of around 6 to 7ma and a closed loop gain of about 8dB or so. But this would be a major project and would involve a complete redesign of both the PC board and parts list. Caps would all have to be upgraded to around 200V or so. I have searched around the web looking for someone that has already done this, but came up empty. Do you have a link to an available design?
Thanks. Looks like the B+ is up in the 250V+ range. Max voltage on the plate of 6AK5 and 6J1 is 180 and 200 in the spec sheets. Can't see the cap voltage ratings in any of the pics. I wonder if they send you a schematic if you select the board only option?
I ordered one of the kits to take a look. Probably be a few weeks to get here. I just wanted to get a closer look for now. Don't know when I'll actually assemble and mess with it. I don't have tools to test the performance besides my ears anyway.
Looking more closely at the pcb photos.. it looks like high voltage input is for a single secondary winding (no center tap) and the silkscreening only says 200vac. Possibly they are creating a virtual center tap but I can't get a good enough view of the parts. Since the heater voltage input is so low (6-9v) I imagine it is running them parallel. It appears to use a regulator on the heaters which would be nice.
Gonna get on my small soap box here for just a minute. Just my opinion of course, and not to poo poo on this thread at all...
There were obviously vacuum tubes designed, back before transistors, to run at low plate to cathode voltages ( ie automobile radios, military walkie talkies, etc ). But in general the whole premise of how tubes work is high voltage and low current. Looking at most tubes' datasheets show that the straightest and most evenly spaced lines occur when moving up those axis. The curves get pretty nasty down in the weeds so to speak.
I understand why these kits are sold. High voltages are dangerous and kinda scary. Powering something off a wall wart is way safer. That is all... off my rickety soap box now
@Kornbread said:
Did order the uTracer and if, big IF, I get it up and running, it should be good for matching tubes working at different voltages. In for $250 on it.
Kornbread, good luck with the uTracer project! In the meantime, while we wait for tubes, I decided to calibrate one of my two 6J1 preamps by matching all the resistors to a very tight tolerance of around 0.1%. Then I re-measured the gain of each tube, swapping them back and forth between the right and left channels, recording the gain, and then averaging the two gain measurements. I then converted this average voltage gain to decibels and added this figure, as a new column, to my testing spreadsheet (attached below). The result is a much more accurate match based on "real circuit" operating conditions. As you can see, your "matched" set of RCA tubes actually shifts the stereo balance by about 0.5dB, which is what you were probably hearing. I could clearly hear this balance shift as well. Your matched set of 6J1P-EV Voskhod Rockets, however, were an almost perfect match with a very small 0.15dB channel to channel difference. The stereo balance when playing music on these seemed very good to my old ears. The only problem I had with the 6J1P-EV Voskhod Rockets was that they seemed very microphonic. They also had a very detailed sound, compared to the RCA's which seemed more laid back and smooth sounding. Also, the RCA's had a very low noise level, even with the volume turned all the way up.
I have also attached a smaller spreadsheet of just the ten OTK stamped tubes that you sent me. I sorted this spreadsheet on the gain column so that we can clearly see the matches. Two of these tubes were a perfect match (7.12 and 7.16dB). Most of the others match up in pairs that differ by only about 0.2 to 0.3dB or so. The accuracy of this matching is much better than using the transconductance column, because these gain (mu) figures are based on the actual gain of the tubes under real operating conditions. When the other tubes come in, I can measure and match them them up in the same way. Later on, when you get your utracer up and running, you can re-test all these tubes and see how they compare to my measurements.
@PWRRYD said:
Gonna get on my small soap box here for just a minute. Just my opinion of course, and not to poo poo on this thread at all...
There were obviously vacuum tubes designed, back before transistors, to run at low plate to cathode voltages ( ie automobile radios, military walkie talkies, etc ). But in general the whole premise of how tubes work is high voltage and low current. Looking at most tubes' datasheets show that the straightest and most evenly spaced lines occur when moving up those axis. The curves get pretty nasty down in the weeds so to speak.
I understand why these kits are sold. High voltages are dangerous and kinda scary. Powering something off a wall wart is way safer. That is all... off my rickety soap box now
Craig, In this particular case, I don't think the curves are quite as nasty as one might think. Go to the on-line "Universal loadline calculator for Vacuum Tubes" found at https://www.vtadiy.com/loadline-calculators/loadline-calculator/ and enter the following:
This generates a loadline for a single stage 6J1 tube operating in triode mode. With a cathode resistor of 200 ohms, the load line predicts a plate voltage of 48V, a plate current of 2.8ma, and a grid voltage Vg of 0.5V. This is very close to my actual measurements of plate = 47.4V, plate current = 2.4ma, and Vg = 0.49V. Notice that the characteristic curve appears fairly linear both above and below this operating point from 30V to 60V. Moving to a higher plate voltage would provide a greater high/low voltage swing, but not a significant improvement in linearity.
@Tom_S said:
So I hooked up the 6J1 to my Pass F6 clone last night and noticed the top end was really rolled off. Time to investigate.
Try swapping out to a different RCA interconnect cable. Capacitance/ft may be too high.
That was the problem! I replaced the old AudioQuest Jade interconnects (circa 1995) from the 6J1 to the F6 with some Apature cables I picked up somewhere and the high end is back.
The above 4 types should cover all the common audio tube pinouts. In addition, if you ever want to build a single ended 807 power amp, you will need a 5 pin ceramic socket together with an insulated, high voltage plate pin connector. Blueglow electronics has put together a single page PDF of all the common audio tube pinouts that you can print and keep handy: https://blueglowelectronics.wordpress.com/diagrams/
uTracer is in the states and tube sockets are ordered. Probably shouldn't have ordered the 4-pin socket as I don't want to risk damaging someone else's expensive 300b. OuCh!
They found the mistake and said they were sending the tube pre kit, but who knows if it will ever show up. I'm certainly not going to try to send something back to China.
It did show up, but they didn't send the output caps. Looks like they spec 2.2uf. Funny enough PE actually carries the identical brand/value cap for $1.19ea.
I mirrored (L/R) the back side image to line up with the top side.
No input caps? Hope a grid doesn't get shorted Otherwise seems like a nice, thoughtful board design.
Looks like this only uses a single DC rail. I'm not seeing a virtual center tap.
Was initially perplexed at the mosfet burried in the high voltage supply. Looks like the gate is connected to a 22uf cap that charges through a 50k resistor.. Power on delay? no way, realy?
Comments
I like the mids and up of the rca pair but the off-center presentation that changes sides when tubes change positions, bothers me, especially when paying a premium for matching.
Anyhow, I've been in contact with UTracer. May be posting a diy tube tracer build before too long.
Tom, you would probably need to put a 1k ohm dropping resistor in series with each input to insure that the mono 400Hz signal coming from the source drops out evenly across both input channels. I tested this and found that this was necessary to insure proper voltage breakouts. I have been using both channels of my oscilloscope to measure and view the relative voltage match at the outputs, but you could also use a pair of DMM's to do this.
I have now done a bunch of transconductance (GM) measurements on my tube tester and I am comparing the results (matches?) with the simple voltage gain matching procedure that you suggested above. And I am coming to the conclusion that transconductance matching is NOT a very good matching procedure at all. You also need to take into account the tubes native voltage gain (mu), and the tube's heater filament resistance. Heater filament resistance must be taken into account because the filaments are connected in series on this particular 6J1 preamp build. One tube could be getting 5.7 volts while the other is getting 6.5 volts. If you have ever tested any tubes on a tester, you know that filament voltage can make a huge difference in the transconductance measurement!! And you don't know this breakout unless you measure the filaments actually connected in series in the preamp. A tube tester does not do this. Your simple, in circuit, voltage gain measurement procedure may actually be the best way to match these tubes. Here are a couple links I found on this subject that reach the same conclusion:
https://tubemaze.info/tube-matching-101/
https://fuzzaudio.com/how-to-interpret-tube-test-results#:~:text=Transconductance and mutual conductance are the same thing,not guarantee the tube will not be microphonic
Strange... This design seems a lot more sensitive to slight tube variances than I'm use to. Grant it I've only built a few preamps, line stages, and power amps. But I've never experienced significant stage shift with tubes not matching perfectly. On the contrary I have had major stage shifts with tweeters that were poorly matched (dB wise). I wonder what makes this circuit design so picky.
Possibly the lower voltages?
Around the 8:20 mark, 3 tubes are tested from 12v - 240v. While they tend to be fairly close at higher voltages, not so much at 12v. Maybe this helps explain why these preamps seem to be so picky about matching?
So I hooked up the 6J1 to my Pass F6 clone last night and noticed the top end was really rolled off. Time to investigate.
I started wondering what it would take to bump up the voltage. Omitting the doubler circuit for starters. It looks like there are other kits out there tailored to higher voltages (PRT-01A). So starting to get out of scope.
On a side note it does look like Zero Zone has someone with some audio experience designing their boards. All the boards I've seen seem to incorporate star grounding for the signal stuff then switch to power plane style design for the power supply sections.
Ya, that could be some of it. > @DrewsBrews said:
You may be onto something about the low voltages. The 6AK5 is designed to operate at a much higher plate voltage (120Vdc or so) and we are only using 60Vdc. Plate current is only about 2.5ma, whereas at higher voltages it is about 7ma. My tube tester is probably measuring the transconductance at a much higher voltage. Best procedure for matching, going forward, will probably be to use a combination of the transconductance measurements AND the actual "in-circuit" gain measurements made at the lower voltages. The utracer that Kornbread is looking at could simultaneously measure both gain and transconductance at the lower voltages by simply programming the curves for the proper set of voltages. My Heathkit tube tester runs the test at one and only one operating point and I cannot adjust this. The problem with the utracer, however, is that construction of a device like this looks like a very advanced technical project with lots of twists and turns and a low probability of success.
I have the 6J1/5654 model set up in LTspice, so I could adjust the plate voltage to 120 and re-set the operating point for an idle current of around 6 to 7ma and a closed loop gain of about 8dB or so. But this would be a major project and would involve a complete redesign of both the PC board and parts list. Caps would all have to be upgraded to around 200V or so. I have searched around the web looking for someone that has already done this, but came up empty. Do you have a link to an available design?
Did order the uTracer and if, big IF, I get it up and running, it should be good for matching tubes working at different voltages. In for $250 on it.
This what you are asking?
https://www.amazon.com/PRT-01A-6J1-Preamp-Finished-Board-Preamplifier/dp/B082M445BD/ref=mp_s_a_1_2?crid=32F4TFMST7NRM&dib=eyJ2IjoiMSJ9.mbUUczhBzhzu-2uVP4CySUQUFiloTCOCjokj2UCNSH3fzFm8sv9_bOimcewFwlpsF9xDX6cK0Kv0nFl_WVowoygIqu2ZOxIY-CJHaknjMQscP-MsweDGTatPdztJNlXyL04jZpvPFz6YyoYzdgEJxOKbX0e4_F73r1JvDj5lE00.p7_pnKpvmEH5YenjEiBqIkHo8_zIZbK0Q4PbwaYsHdA&dib_tag=se&keywords=prt-01a&qid=1709829377&sprefix=prt-01a,aps,336&sr=8-2
Also found the kit on AliExpress for $21 shipped free
https://www.aliexpress.us/item/3256806251614199.html?spm=a2g0n.productlist.0.0.29673e7aP83kEV&browser_id=1201889cdf104b749de4b13b2b622884&aff_platform=msite&m_page_id=wjhinhbhubasyaea18e19c4dc1480351bf22099782&gclid=&pdp_npi=4@dis!USD!24.99!20.99!!!24.99!20.99!@2101ea8c17098293155813272ed083!12000037166928956!sea!US!2985515921!&algo_pvid=8a829270-3bb0-4c71-ab0b-0eeb6f23b90e
Or if ya just want the pcb
https://www.amazon.com/Jolooyo-PRT-01A-6J1-Tube-Preamp-PCB/dp/B07BK42N9S/ref=mp_s_a_1_1?crid=32F4TFMST7NRM&dib=eyJ2IjoiMSJ9.mbUUczhBzhzu-2uVP4CySUQUFiloTCOCjokj2UCNSH3fzFm8sv9_bOimcewFwlpsF9xDX6cK0Kv0nFl_WVowoygIqu2ZOxIY-CJHaknjMQscP-MsweDGTatPdztJNlXyL04jZpvPFz6YyoYzdgEJxOKbX0e4_F73r1JvDj5lE00.p7_pnKpvmEH5YenjEiBqIkHo8_zIZbK0Q4PbwaYsHdA&dib_tag=se&keywords=prt-01a&qid=1709830008&sprefix=prt-01a,aps,336&sr=8-1
Thanks. Looks like the B+ is up in the 250V+ range. Max voltage on the plate of 6AK5 and 6J1 is 180 and 200 in the spec sheets. Can't see the cap voltage ratings in any of the pics. I wonder if they send you a schematic if you select the board only option?
I ordered one of the kits to take a look. Probably be a few weeks to get here. I just wanted to get a closer look for now. Don't know when I'll actually assemble and mess with it. I don't have tools to test the performance besides my ears anyway.
Looking more closely at the pcb photos.. it looks like high voltage input is for a single secondary winding (no center tap) and the silkscreening only says 200vac. Possibly they are creating a virtual center tap but I can't get a good enough view of the parts. Since the heater voltage input is so low (6-9v) I imagine it is running them parallel. It appears to use a regulator on the heaters which would be nice.
That sounds about right. Good luck with the project.
Try swapping out to a different RCA interconnect cable. Capacitance/ft may be too high.
Gonna get on my small soap box here for just a minute. Just my opinion of course, and not to poo poo on this thread at all...
There were obviously vacuum tubes designed, back before transistors, to run at low plate to cathode voltages ( ie automobile radios, military walkie talkies, etc ). But in general the whole premise of how tubes work is high voltage and low current. Looking at most tubes' datasheets show that the straightest and most evenly spaced lines occur when moving up those axis. The curves get pretty nasty down in the weeds so to speak.
I understand why these kits are sold. High voltages are dangerous and kinda scary. Powering something off a wall wart is way safer. That is all... off my rickety soap box now
Kornbread, good luck with the uTracer project! In the meantime, while we wait for tubes, I decided to calibrate one of my two 6J1 preamps by matching all the resistors to a very tight tolerance of around 0.1%. Then I re-measured the gain of each tube, swapping them back and forth between the right and left channels, recording the gain, and then averaging the two gain measurements. I then converted this average voltage gain to decibels and added this figure, as a new column, to my testing spreadsheet (attached below). The result is a much more accurate match based on "real circuit" operating conditions. As you can see, your "matched" set of RCA tubes actually shifts the stereo balance by about 0.5dB, which is what you were probably hearing. I could clearly hear this balance shift as well. Your matched set of 6J1P-EV Voskhod Rockets, however, were an almost perfect match with a very small 0.15dB channel to channel difference. The stereo balance when playing music on these seemed very good to my old ears. The only problem I had with the 6J1P-EV Voskhod Rockets was that they seemed very microphonic. They also had a very detailed sound, compared to the RCA's which seemed more laid back and smooth sounding. Also, the RCA's had a very low noise level, even with the volume turned all the way up.
I have also attached a smaller spreadsheet of just the ten OTK stamped tubes that you sent me. I sorted this spreadsheet on the gain column so that we can clearly see the matches. Two of these tubes were a perfect match (7.12 and 7.16dB). Most of the others match up in pairs that differ by only about 0.2 to 0.3dB or so. The accuracy of this matching is much better than using the transconductance column, because these gain (mu) figures are based on the actual gain of the tubes under real operating conditions. When the other tubes come in, I can measure and match them them up in the same way. Later on, when you get your utracer up and running, you can re-test all these tubes and see how they compare to my measurements.
Craig, In this particular case, I don't think the curves are quite as nasty as one might think. Go to the on-line "Universal loadline calculator for Vacuum Tubes" found at https://www.vtadiy.com/loadline-calculators/loadline-calculator/ and enter the following:
Select a tube: 6J1
Operating mode: Triode
V+(V): 60
Load (ohm) Resistive: 4700
Next stage AC impedance (ohm): 22,000
This generates a loadline for a single stage 6J1 tube operating in triode mode. With a cathode resistor of 200 ohms, the load line predicts a plate voltage of 48V, a plate current of 2.8ma, and a grid voltage Vg of 0.5V. This is very close to my actual measurements of plate = 47.4V, plate current = 2.4ma, and Vg = 0.49V. Notice that the characteristic curve appears fairly linear both above and below this operating point from 30V to 60V. Moving to a higher plate voltage would provide a greater high/low voltage swing, but not a significant improvement in linearity.
That was the problem! I replaced the old AudioQuest Jade interconnects (circa 1995) from the 6J1 to the F6 with some Apature cables I picked up somewhere and the high end is back.
I guess I can't argue with an online "Universal loadline calculator for Vacuum Tubes".
Should have room for 4-5 tube bases in the uTracer build. Miniature 7-pin and Noval 9-pin, what other tube bases to cover most audio tubes?
An octal base for EL34/6550/6L6 types.
A 4 pin for the Single Ended Triode fans!
The above 4 types should cover all the common audio tube pinouts. In addition, if you ever want to build a single ended 807 power amp, you will need a 5 pin ceramic socket together with an insulated, high voltage plate pin connector. Blueglow electronics has put together a single page PDF of all the common audio tube pinouts that you can print and keep handy:
https://blueglowelectronics.wordpress.com/diagrams/
I don't know how many folks are building it but Pete Millet's "Engineer's Amp" uses 12 pin compactron tubes for outputs...
Everything I've ever built was standard 9 pins and 8 pin octal sockets.
uTracer is in the states and tube sockets are ordered. Probably shouldn't have ordered the 4-pin socket as I don't want to risk damaging someone else's expensive 300b. OuCh!
Welp they accidentally sent a headphone amp kit instead
(https://www.aliexpress.us/item/2255801113124673.html?src=google&aff_fcid=684fefbbca1941dcbc46c4bf73bc9646-1711149056534-06958-UneMJZVf&aff_fsk=UneMJZVf&aff_platform=aaf&sk=UneMJZVf&aff_trace_key=684fefbbca1941dcbc46c4bf73bc9646-1711149056534-06958-UneMJZVf&terminal_id=adf3cb7420fa473dac73ee341f515fc7&afSmartRedirect=y&gatewayAdapt=glo2usa)
They found the mistake and said they were sending the tube pre kit, but who knows if it will ever show up. I'm certainly not going to try to send something back to China.
I have a similar china headphone amp and it sounds great.
It did show up, but they didn't send the output caps. Looks like they spec 2.2uf. Funny enough PE actually carries the identical brand/value cap for $1.19ea.
I mirrored (L/R) the back side image to line up with the top side.
No input caps? Hope a grid doesn't get shorted
Otherwise seems like a nice, thoughtful board design.
Looks like this only uses a single DC rail. I'm not seeing a virtual center tap.
Was initially perplexed at the mosfet burried in the high voltage supply. Looks like the gate is connected to a 22uf cap that charges through a 50k resistor.. Power on delay? no way, realy?